a jelly roll is (generally) 40 strips, 2 1/2" wide by width of fabric (40"-42") long...some jelly rolls have fewer strips.
so, it depends on how large you want your lap quilt- if you stitched 2 strips together on the short end to give yourself long strips (about 78" after removing salvages) you would have 20 strips x 2" (the 1/2" being seam allowance) so 40" wide by 78" long from a 40 strip jelly roll.